Definition:
Addition:
(TapaniRaiko: The proposal is not enough, as these two examples show. I have seen an example where a 35x35 board was filled with small groups all living in a single seki.)
How about something along the lines of:
"A local position with at least one mutual liberty or single point ko which cannot be filled by either side without incurring a loss." We still have to read out the consequences of moves, but I think this captures what we mean by Seki and applies to all the examples there.
jvt: This is still ambiguous: how to define which chains belong to the 'local position'? Some chains may be in seki even if they are not adjacent to any dame.
RobertJasiek: Needless to say, the Japanese 2003 Rules will provide an unequivocal definition of "seki" under those rules. For other rules, minor modifications would be necessary then.
KjeldPetersen: If a group (with max 1 eye) is moving out (not filling the eye) and thereby get into a position of atari, then this group is posible part of a seki? Look at the example at the top. All three groups will go into atari if they move out.